Low-voltage fuses with indicators



March 6, 1956 P, c. HITCHCOCK 2,737,552

OW-VOLTAGE FUSES WITH INDICATORS Filed Dec. 21, 1954 2 Sheets-Sheet l March 1956 P. c. HITCHCOCK LOW-VOLTAGE FUSES WITH INDICATORS 2 Sheets-Sheet 2 Filed Dec. 21, 1954 United States Patent LOW-VOLTAGE FUSES WITH INDICATORS Paul C. Hitchcock, Groveland, Mass, assignor to The Chase-Shawmut Company, Newburyport, Mass.

Application December 21, 1954, Serial No. 476,753 4 Claims. (Cl. 296-421) This invention relates to cartridge fuses comprising means for indicating blowing of the fuse.

It is one object of the invention to provide a cartridge fuse for relatively low circuit voltages having inexpensive means for giving a readily visible indication of whether or not the fuse has blown.

Fuses having shunt links for indicating purposes are generally relatively complicated structures and include holes in their casing for the passage of the indicator link from the inside of the casing to the outside thereof. Provision of such passages is generally objectionable because they reduce the mechanical strength of the casing and tend to permit escape of hot products of arcing.

It is therefore another object of the invention to provide a cartridge fuse having an indicator link wholly arranged on the outside of the casing, the presence of which link does not affect the mechanical integrity of the casmg.

Still another object of the invention is to provide a cartridge fuse comprising a blown fuse indicator which is predicated upon the condensation of products of arcing on a normally transparent or light-transmitting surface.

Further objects and advantages of this invention will become apparent as the following description proceeds, and the features of novelty which characterize the invention will be pointed out with particularity in the claims annexed to and forming part of this specification.

For a better understanding of this invention reference may be had to the accompanying drawing which illustrates several embodiments of the invention and wherein:

Fig. 1 is a side elevation of a fuse embodying my invention upon removal of the outer top layer of the structure;

Fig. 2 is a top plan view of the structure shown in Fig. l with some of the constituent parts thereof-including the outer top layer-broken away;

Fig. 3 is a cross-section along 33 of Fig. 2;

Fig. 4 is a top plan view of another fuse embodying my invention with some of the constituent parts thereof broken away;

Fig. 5 is a section along 5-5 of Fig. 4, and

Fig. 6 is a section along 6-6 of Fig. 5.

Referring now to Figs. 1 to 3, inclusive, reference numeral 1 has been applied to indicate a substantially tubular casing of insulating material. The casing may, for instance, consist of fiber, but I prefer to use an insulating material which has more dimensional stability than fiber as, for instance, a synthetic-resin-glass-cloth-larninate. Each of the two terminal caps 2 is mounted under considerable pressure on one end of easing 1. The axially inner ends of caps 2 may be crimped and may be caused to enter a pair of grooves provided in casing 1 to attach caps 2 firmly to the casing. Caps 2 are provided with blade contacts 2a for insertion of the fuse into a fuse holder and an electric circuit. Fuse link means 3 having a relatively high current-carrying capacity arranged within casing 1 interconnect conductively both terminal caps 2. Fuse link means 3 may consist of one single 2,737,552 Patented Mar. 6, 1956 conductor or of a plurality of conductors establishing a plurality of parallel current paths from one cap 2 to the other. If desired fuse link means 3 may be immersed in a pulverulent arc quenching filler, e. g. silica. In addition to fuse link means 3 the fuse is provided with an indicator fuse link 4 which has a very limited currentcarrying capacity. The cross-sectional area of indicator fuse link 4 is much smaller than that of use link means 3 within casing 1 and indicator fuse link 4 is arranged wholly outside of easing 1. Link 4 is conductively connected to caps 2 by means of joints of soft solder. A strip 5 of insulating material wound around casing 1 occupies the toroidal space bounded by easing 1 and caps 2. Strip 5 forms a gap '7 between the ends thereof accommodating indicator link 4. The transparent insulating film 6 covers link 4 and gap '7. The opaque insulating envelope 9 is wrapped around caps 2 and strip 5 and has an aperture 8 substantially in registry with the center portion of link a, i. e. the portion thereof situated halfway between caps 2. Envelope 9 may consist of paper, be glued to strip 5 and caps 2, and be used as a label for indicating the rating of the fuse and the make thereof. Aperture 8 in insulating sheet or envelope 9 is surrounded by a dark field 9', e. g. a black field, to increase visual contrast between sheet or envelope 9 and indicator link 4 prior to blowing thereof.

The device shown in Figs. 1 to 3, inclusive, operates as follows: Normally the entire current is carried by the link means 3 within casing 1. There is virtually no difference in potential between the two terminal caps 2. On oc currence of an overcurrent fuse link means 3 fuse and vaporize under the heat of the are incident upon interruption of the current path through fuse link means 3. Upon extinction of the arc, a gaseous insulating gap takes the place of link means 3, and then there tends to be a difference of potential between the caps 2. As a result, a current flows through indicator link 4 which is thus caused to fuse or blow. The empty space under link 4 forms a thermal insulator causing the highest temperature along link 4 to occur at the point where link 4 is thermally insulated by the empty space which is situated under it. Hence blowing of link 4 will occur substantially at the center thereof. The products of arcing resulting from vaporization of use link 4 will condense on the inside of transparent film 4 and the condensation products formed will be readily visible through the circular aperture 8 in envelope 9. The strip 5 of insulating material has the dual function of providing a support for envelope 9 and of restraining diffusion of the products of arcing formed upon fusion of link 4 away from the region of aperture 8. In other words, strip 5 serves as a barrier restraining the flow of products of arcing formed upon vaporization of link i away from the region of aperture 8. Consequently, there will be a relatively intense condensation of products of arcing on the inside of film 6, resulting in an intense discoloration thereof which is readily perceptible.

The indicator link 4 consists of a very fragile metal foil, e. g. a zinc strip .001 x .053 in. in size. The required exact determination of the point of arc initiation is achieved by the provision of the aforementioned empty space from where heat fiow away from link 4 is minirnized and thus the rate of rise of temperature highest. This empty space serves also as an arcing chamber of judiciously predetermined volume to obtain an optimum of precipitation or condensation of products of arcing on the transparent film window 6.

Referring now to Figs. 4 to 6, inclusive, numeral 15 has been applied to indicate a substantially tubular casing of a heat shock resistant, dimensionally stable insulating material, such as the above mentioned synthetic-resinglass-cloth-laminate. The fuse comprises a pair of terminal elements which include cylindrical copper blocks 16 inserted into casing 15 and blade contacts 17 projecting from blocks 16. Fuse link means 18 having a relatively high current-carrying capacity arranged Within casing 15 interconnect conductively the two terminal blocks 16. For the purpose of illustration fuse link means 18 have been shown as consisting of two ribbon-type fuse links having a reduced cross-section portion or neck situated between the terminal blocks l6. If desired, fuse link means 18 may be immersed in a pulverulent arc-quenching medium 14. The fuse link 1% arranged outside of casing 25 is also of the ribbon-type, yet it is a link having a relatively limited current-carrying capacity, i. e. having a high resistance. Casing 15 is provided with a recess 20 which is arranged in registry with the center of link 19 and determines the point of initial fusion thereof by establishing a restricted zone of limited heat exchange. Two bores 21 project transversely across casing 15 into terminal blocks 16. An eyelet 22 is inserted into each bore 21 and each end of indicator link 19 is inserted into the hollow space defined by one of the eyelets 22. Drops of solder 23 placed into each eyelet 22 establish a current conducting bond between the eyelets 22 and the link 19. The link means 18 are thus shunted by a current-path extending from one of the terminal blocks 16 through one of the eyelets 22 and the fuse link 19 to the other of the eyelets 22 and the other of the terminal blocks 16. A transparent insulating film 24 made, for instance, of acetyl cellulose or another suitable plastic, covers the center portion of indicator link 19 and recess 2%) in casing 15. The opaque insulating sheet 25 covers film 24 and is wound around the entire periphery of and glued or pasted to casing 15. Insulating sheet 25 may be used as a label indicating the make of the fuse, the rating thereof, etc. Insulating sheet 25 has an opening 26 which is arranged substantially in registry with recess 2% in casing 15' and with the center region of link 19. i

1 Link 19 may consist of zinc which reflects light incident upon it fairly Well. Hence the space in back of opening 26 in insulating sheet 25 will normally appear fairly bright. To increase the brightness efiect of link 19 by way of visual contrast the surface of sheet 25 immediately adjacent to opening 26 is preferably colored dark, e. g. blackened by printing on it a circular field 27 surrounding the circular opening 25.

Since indicator link 19 is not enclosed in a pressureresistant enclosure, nor surrounded by efiective de-ionizing means, structures of the kind shown in Figs. 4 to 6- as well as of the kind shown in Figs. 1 to 3do not lend themselves to applications where the system voltage and the transient recovery voltage are relatively high. Where the system voltage is low, say in the order of 65 volts, the devices disclosed perform their duty effectively and consistently in spite of the relatively simple means on which they rely for indication of blowing.

Having disclosed several preferred embodiments of my invention, it is desired that the same be not limited to the particular structures disclosed. It will be obvious to any person skilled in the art that many modifications and changes may be made without departing from the broad spirit and scope of the invention. Therefore it is desired that the invention be interpreted as broadly as possible and that it be limited only as required by the prior state of the art.

I claim as my invention:

1. A cartridge fuse for low circuit voltages comprising a substantially tubular casing of insulating material having openings for the passage of fuse link means only at the axially outer ends thereof to maintain the mechanical strength of said casing and to preclude lateral venting of are products, terminal elements arranged at the axially outer ends of said casing closing said casing, a first fuse link means having a relatively high current-carrying capacity arranged inside of said casing conductively interconnecting said terminal elements, a second fuse link means having a relatively small current-carrying capacity arranged in the entirety thereof outside of said casing conductively interconnecting said terminal elements, a transparent insulating film covering said second fuse link means, and an opaque insulating sheet covering said film and having an aperture substantially in registry with the center portion of said second fuse link means.

2. A cartridge fuse according to claim 1 wherein said aperture in said insulating sheet is surrounded by a dark field to increase visual contrast between said insulating sheet and said second fuse link means prior to blowing thereof.

3. A cartridge fuse comprising a substantially tubular casing of insulating material, a pair of terminal caps each mounted on one end of said casing, a first ribbon-type fuse link having a relatively large current-carrying capacity arranged within said casing and conductively interconnecting said pair of caps, a second ribbon-type fuse link having a relatively small current-carrying capacity arranged wholly outside of in spaced relation with respect to said casing and conductively interconnecting said pair of caps, a strip of insulating material wound around said casing forming a gap between the ends thereof accommodating said second link, a transparent insulating film covering said second link and said gap, and an opaque insulating envelope wrapped around said pair of caps and said strip pasted to said strip and having an aperture substantially in registry with the center portion of said second link.

4. A cartridge fuse comprising a substantially tubular casing of insulating material, a pair of terminal elements inserted into the axially outer ends of said casing to close said ends, fuse link means having a relatively high currentcarrying capacity arranged within said casing conductively interconnecting said pair of terminal elements, a pair of bores each extending transversely through said casing and into one of said pair of terminal elements, a pair of eyelets each in one of said pair of bores and each in conductive relation with one of said pair of terminal elements, a fuse link having a relatively small current-carrying capacity arranged in the entirety thereof on the outside of said casing with each end thereof inserted into one of said pair of eyelets and conductively connected thereto, a recess in said casing situated under said link substantially in registry with the center portion thereof, a transparent insulating film covering said link, and an opaque insulating sheet covering said film and having an aperture substantially in registry with said recess and said center portion of said link.

References Cited in the file of this patent UNITED STATES PATENTS 953,204 Hoffmann Mar. 29, 1910 FOREIGN PATENTS 692,967 Germany June 29, 1940 

